1. Field of the Invention
The present invention relates to a low melting point glass suitable for covering for insulation of transparent electrodes of e.g. ITO (indium oxide doped with tin) or tin oxide, and a plasma display device.
2. Discussion of Background
In recent years, a thin flat plate type color display device has attracted an attention. In such a display device, it is necessary to form an electrode for each pixel in order to control the display state in the pixel for forming an image. In order to prevent deterioration of the image quality, transparent electrodes are used as such electrodes. As the transparent electrodes, thin films of ITO or tin oxide formed on a glass substrate, are commonly employed. Here, the tin oxide includes tin oxide doped with e.g. fluorine or antimony.
Transparent electrodes which are formed on the surface of a glass substrate to be used as a display panel of the above display device, are formed into fine lines to realize fine images. In order to control the respective pixels independently, it is necessary to secure insulation among such finely formed transparent electrodes. However, if moisture is present on the surface of the glass substrate, or if an alkali component is present in the glass substrate, it may happen that an electrical current flows to some extent via the surface of this glass substrate. To prevent such a current, it is effective to form an insulating layer between the transparent electrodes. Further, in order to prevent deterioration of the image quality by the insulating layer formed between the transparent electrodes, such an insulating layer is preferably transparent.
Various materials are known as an insulating material for forming such an insulating layer. Among them, a glass material is widely employed which is a transparent and highly reliable insulating material.
In a plasma display device (hereinafter referred to as PDP) which is recently expected as a large size flat color display device, cells are defined and formed by a front substrate used as a display surface, a rear substrate and barrier ribs, and an image will be formed by generating plasma discharge in the cells. Transparent electrodes are formed on the surface of the front substrate, and it is essential to cover the transparent electrodes with a glass excellent in plasma durability in order to protect the transparent electrodes from plasma.
Such a glass to be used for covering electrodes, is employed usually in the form of a glass powder. Namely, to such a glass powder, a filler, etc. may be added as the case requires, and then the mixture is formed into a paste. A glass paste thus obtained is coated on a glass substrate having transparent electrodes preliminarily formed, followed by firing to cover the transparent electrodes.
Such a glass for covering electrodes, is required to have an electrical insulating property, and it is further required that the softening point is, for example, at most 650xc2x0 C., the linear expansion coefficient is, for example, about 80xc3x9710xe2x88x927/xc2x0 C., and transparency of the glass layer covering the electrodes, obtainable by firing, is high. Various glasses have been proposed. For example, JP-A-11-180726 discloses a non-crystalline glass consisting essentially of from 52 to 86% of PbO+Bi2O3, from 14 to 28% of B2O3, from 0 to 5% of SiO2, from 6 to 23% of ZnO, from 0 to 8% of Al2O3, from 0 to 5% of CeO2 and from 0 to 5% of SnO2, as represented by mass percentage.
However, for PDP, a further improvement of image quality has been desired in recent years. Accordingly, it is required to further improve the transparency of the glass layer covering electrodes.
It is an object of the present invention to provide a low melting point glass for covering electrodes and a plasma display device, which will solve such problems.
The present invention provides a low melting point glass for covering electrodes, which contains Cu in a content as calculated as CuO within a range of from 0.1 to 0.9% by mass percentage and which contains neither Mo nor Sb.
The present invention also provides a low melting point glass for covering electrodes, which contains Cu in a content as calculated as CuO within a range of from 0.1 to 0.9% by mass percentage and which contains at least one of Mo and Sb, wherein the total of the content of Cu as calculated as CuO, the content of Mo as calculated as MoO3 and the content of Sb as calculated as Sb2O3 is within a range of from 0.2 to 1.4% by mass percentage.
Further, the present invention provides a plasma display device having a front substrate, wherein transparent electrodes formed on a glass substrate constituting the front substrate are covered by such a low melting point glass for covering electrodes.
The present inventors have considered that one of the causes for the decrease in the transparency of the glass layer covering electrodes in PDP is attributable to retention of the after mentioned carbon-containing impurities in the glass layer covering electrodes, and have arrived at the present invention based on this assumption.
The low melting point glass for covering electrodes is used usually in the form of a powder. The powder of the low melting point glass for covering electrodes is formed into a glass paste by using an organic vehicle or the like to impart printability, and such a glass paste is coated on electrodes formed on a glass substrate, followed by firing to cover the electrodes.
The glass layer covering electrodes, obtained by firing, is colored brown or black in many cases even if a coloring component such as a transition metal, is not contained. This phenomenon is considered to be such that carbon-containing impurities contained in the organic vehicle or the like remain in the glass layer covering electrodes, and the glass layer covering electrodes is colored by such carbon-containing impurities. By the above coloring with the brown color, the transmittance of light having a wavelength of 400 nm typically decreases. Further, it is considered that when plasma is generated in PDP, such carbon-containing impurities react with water or the like present in the glass layer covering electrodes and will be discharged from the glass layer covering electrodes, whereby brightness of PDP will also decrease.